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OLIVE OIL

pid oxidation in its initial stages. However, they cannot block the autocatalytic mechanism. This effect is clearly visible from the data described by Bendini et al. (2006) for an accelerated oxidation test (60°C) on virgin olive oils which differed only with respect to phenol contents. During the six weeks of tEsting, the peroxide and oxidized fatty acid (OFA) values increased in all the oils. There was a contemporary decrease of oxidative stability (OSI time), but with different rates that were proportional to the initial content in phenolic antioxidants. This effect was amplified when copper was added as a catalyst of lipid oxidation. Mancebo-Campos et al. (2007) studied the oxidation process of several virgin olive oils with different contents of natural antioxidants under accelerated oxidation conditions (Rancimat) and compared to long-term storage at room temperature. All the samples reached the peroxide number limit (20 meq O2 kg-1) but in very different times (28-56 hours vs 96-167 weeks) depending on the temperature (100°C for the Rancimat test and 25°C for the prolonged storage) and the initial content in antioxidants. Tocopherols Although α-tocopherol is considered to be the most relevant antioxidant in vegetable oils, as well as in the protection of the lipid structures in vivo, several researchers have reported a lower antioxidant activity than hydroxytyrosol (Le Tutour and Guedon, 1992; Baldioli et al., 1996; Mateos, 2002). This may be explained by the “antioxidant polarity paradox” which states

that hydrophilic antioxidants are often less effective in oil-in-water emulsions than lipophilic antioxidants, whereas lipophilic antioxidants are less effective in bulk oils than hydrophilic antioxidants (Porter et al., 1989; Frankel et al., 1994). Moreover, it has also been reported that in the presence of o-diphenols, α-tocopherol gives rise to a synergic effect (Servili et al., 1996). Pigments Carotenoids, especially β-carotene are efficient VOO protectors against photo-oxidation, since they are capable of deactivating the oxygen singlet giving back its triplet status (Cuppett et al., 1997; Wagner and Elmafda, 1999). On the other hand, the capacity of the chlorophyll molecule to absorb light energy and transfer it to chemical substances, makes it very active in lipid photo-oxidation in VOO (Rahmani and Saari-Csallany, 1998). Chlorophylls may also act as low antioxidants during oxidation in the dark – absence of light – probably due to its capacity to donate hydrogen (Endo et al., 1984; Gutiérrez et al., 1992; Psomiadou and Tsimidou, 2002). Filtered vs. unfiltered VOO The results of some studies have hown a gradual loss in stability during the storage of filtered oils mainly due to a significant decrease in the phenolic components. When unfiltered and the corresponding filtered virgin olive oils were stored for nine months at ambient temperature in the dark, a loss in oxidative stability in the latter was observed due to a lower total phenolic content (Tsimidou et al., 2005). Other

researchers (Gómez-Caravaca et al., 2007) have reported that eight virgin olive oils after filtration through cotton in the laboratory showed a significant loss of hydroxytyrosol, a simple phenol Endowed with high antioxidant activity. Consequently there was lower oxidative stability of the filtered oils than of the unfiltered ones. Generally, the formation of simple phenols, such as hydroxytyrosol and tyrosol, was greater in unfiltered olive oils due to the hydrolysis rate of their secoiridoid derivatives. These reactions appear linked to the presence of a higher content of dispersed water droplets that maintain a partial enzymatic activity. On the other hand, it is well known that the filtration step which removes the organic sediments, prevents anaerobic fermentation which produces unpleasant volatile components responsible for the muddy defect. At the same time it has also been reported that filtration and dehydration decrease the hydrolysis rate of the triacylglycerol matrix, especially during storage at the higher temperature (40°C) and in oils with a higher initial free acidity (e.g. free acidity > 0.6%). Moreover, the formation rate of simple phenols due to hydrolysis of their secoiridoid derivatives was also greater in unfiltered olive oils. Thus, from this point of view, filtration and especially dehydration could help prolong the shelf life of some high-quality but less stable virgin olive oils, (e.g. Arbequina and Colombaia varieties) (Fregapane et al., 2006). Degradation of minor constituents in VOO Another consequence of the

Italian Food & Beverage Technology - LX (2010) march -

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ITALIAN BEVERAGE TECHNOLOGY 60/2010  

Rivista esclusivamente in inglese, sviluppata a supporto di tutte quelle aziende italiane che vogliono puntare sull’export di macchine, prod...

ITALIAN BEVERAGE TECHNOLOGY 60/2010  

Rivista esclusivamente in inglese, sviluppata a supporto di tutte quelle aziende italiane che vogliono puntare sull’export di macchine, prod...